troubleshooting, testing, & analysis

COATING THICKNESS MEASUREMENT

DAVID BEAMISH

DEFELSKO CORP., OGDENSBURG, N.Y.

A variety of recognized methods can be used to determine the thickness oforganic coatings. The method employed in a specific situation is most oftendetermined by the type of coating and substrate, the thickness range of thecoating, the size and shape of the part, and economics. Commonly used measuringtechniques are (1) nondestructive dry-film methods such as magnetic,eddy current,ultrasonic, or micrometer measurement; (2) destructive dry-filmmethods such as cross sectioning or gravimetric (mass) measurement; and (3)wet-film measurement.

MAGNETIC

The magnetic technique is used to nondestructively measure the thickness ofnonmagnetic coatings on ferrous substrates. Most coatings on steel and ironare measured this way. Gauges use one of two principles of operation: magneticpull-off or magnetic/electromagnetic induction.

Magnetic Pull-Off

These gauges consist of a permanent magnet, a calibrated spring, and a graduatedscale. The attractive force between the magnet and the magnetized steel pulls thetwo together. As the coating thickness separating the two increases, it becomeseasier to pull the magnet away. Coating thickness is determined by measuring thispull-off force. The weaker the force, the thicker the coating. Testing is sensitive tosurface roughness, curvature, substrate thickness, and alloy content.Magnetic pull-off gauges are rugged, simple, inexpensive, portable, andusually do not require any calibration adjustment. They are the instrumentsof choice when a low number of readings per day is required. Pull-off gaugesare typically classified as either pencil-type or roll-back dial models.Pencil-type models mount a magnet to a helical spring (see Fig. 1). Thespring acts perpendicularlyto thesurface to pull themagnet off. Mosthave large magnetsand are designedto work in onlyone or two positions,which partiallycompensatefor gravity. A moreaccurate versionis available, whichhas a tiny, precisemagnet to measure on small, hot,

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Fig.1 Pencil-type magnetic pull-off thickness gauge.

or hard-to-reach surfaces.A triple indicator ensuresaccurate measurementswhen the gauge is pointeddown, up, or horizontallywith a tolerance of ±10%.Roll-back dial modelsare the most commonform of magnetic pull-offgauge (see Fig. 2). A magnetis attached to one endof a pivoting balanced arm.This assembly is connectedto a calibrated hairspring.By rotating the dial with a finger, the spring increases the force on the magnetand pulls it from the surface. These gauges are easy to use and have a balancedarm, which allows them to work in anyposition, independent of gravity. They aresafe inexplosive environments and are commonly used by painting contractors and small powder coating operations. Typical tolerance is ±5%

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Fig. 2 Roll-back dial magnetic pull-off thickness gauge

Magnetic and Electromagnetic Induction

These electronic instruments measure the change in magnetic flux density at thesurface of a magnetic probe as it is brought near steel. The magnitude of the fluxdensity at the probe surface is directly related to the distance from the steel substrate.By measuring flux density the coating thickness can be determined.Magnetic induction instruments use a permanent magnet as the source ofthe magnetic field. A Hall-effect generator or magneto resistor is used to sensethe magnetic flux density at a pole of the magnet. Electromagnetic inductioninstruments use an alternating magnetic field. A soft, ferro magnetic rod woundwith a coil of fine wire is used to produce a magnetic field. A second coil of wireis used to detect changes in magnetic flux.Electronic magnetic gauges come in all shapes and sizes (see Fig.3). Theycommonly use a constant pressure probe to remove operator influence andquickly display results on a liquid crystal display (LCD). Optionally, they canstore measurement results, perform instant analysis of readings on their displays,and output results to a printer or computer for further examination.Typical tolerance is ±1%.The manufacturer’s instructions should be carefully followed for most accurateresults. Standard methods for the application and performance of this testare available in ASTM D 1400, ISO 2360, and ISO 2808.

EDDY CURRENT

This technique is used to nondestructively measure the thickness of nonconductivecoatings on nonferrous metal substrates. Paint on aluminum andacrylic on copper are typical examples. Eddy current inspection is based on theprinciples of electromagnetic induction and, therefore, has many similaritiesto the electromagnetic induction test method. A coil of fine wire conductinga high-frequency alternating current (above 1 MHz) is used to set up analternating magnetic field at the surface of the instrument’s probe. When theprobe is brought near a conductive surface, the alternating magnetic field will set up eddy currents on thesurface. The magnitude ofthe eddy currents is relatedto substrate characteristicsand the distance,or coatingthickness, from the probe.The eddy currents createtheir own opposing electromagneticfield that canbe sensed by the excitingcoil or by a second, adjacentcoil.Eddy current coatingthickness gauges look andoperate like electronicmagnetic gauges. They areused to measure coatingthickness over all metalsbut steel and iron. Theycommonlyuse a constantpressure probe and displayresults on an LCD.Optionally, they can storemeasurement results or perform instant analysis of readings and output toa printer or computer for further examination. The typical tolerance is ±1%.Testing is sensitive to surface roughness, curvature, substrate thickness, typeof metal substrate, and distance from an edge.

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Fig. 3 Electronic magnetic induction thickness gauges.

most accurate results. Standard methods for the application and performanceof this test are available in ASTM B 244 and ASTM D 1400.It is now commonfor gauges to incorporate both magnetic and eddy current principles into oneunit. Some simplify the task of measuring most coatings over any metal byswitching automatically from one principle of operation to the other, dependingupon the substrate. These combination units are popular with painters andpowder coaters.

ULTRASONIC

The ultrasonic pulse-echo technique is used to nondestructively measure thethickness of various coatings on nonmetal substrates. Applications include painton plastic, lacquer on wood, and epoxy on concrete.The probe of the instrument contains an ultrasonic transducer that sendsa pulse through the coating (see Fig. 4). The pulse reflects back from thesubstrate to the transducer and is converted into a high-frequency electricalsignal. The echo wave form is then digitized and analyzed to determine coatingthickness. In some circumstances, individual layers in a multilayer systemcan be measured.Couplant is typically used between the probe and the surface to be measured.Typical tolerance is ±3%. The manufacturer’s instructions should be carefullyfollowed for most accurate results. Standard methods for the application andperformance of this test are available in ASTM D 6132.

MICROMETER

Micrometers aresometimes usedto check coatingthickness. Theyhave the advantageof measuring anycoating/substratecombination butthe disadvantageof requiring accessto the bare substrate.The requirementto touchboth the surfaceof the coating andthe under side ofthe substrate canbe limiting, and they are often not sensitive enough to measure thin coatings.Two measurements must be taken: one with the coating in place and the otherwithout. The difference between the two readings, the height variation, is taketo be the coating thickness. On rough surfaces, micrometers measure coatingthickness above the highest peak.

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Fig. 4. Ultrasonic gauge can measure the thickness of coatings on nonmetallic

substrates.

CROSS SECTIONING

Coating thickness can be measured by cutting the coated part and viewingthe cut microscopically. It can also be determined by making a geometricallydesigned incision through the dry-film coating and viewing it cross sectionallywith a scaled microscope. A special cutting tool is used to make a small,precise V-groove through the coating and into the substrate. Gauges are availablethat come complete with cutting tips and illuminated scaled magnifier.While the principles of this destructive method are easy to understand, thereare opportunities for measuring error. It takes skill to prepare the sample andinterpret the results. Adjusting the measurement reticule to a jagged or indistinctinterface is a source of inaccuracy, particularly between different operators;however, direct observation of these conditions is sometimes informative. Thismethod is used when inexpensive, nondestructive methods are not possible, oras a way of confirming nondestructive results. ASTM D 4138 outlines a standardmethod for this measurement system.

GRAVIMETRIC

By measuring the mass and area of the coating, thickness can be determined.The simplest method is to weigh the part before and after coating. Once themass and area have been determined, the thickness is calculated using thefollowing equation:T = (m x 10)/(A x d)where T is the thickness in micrometers, m is the mass of the coating in milligrams,A is the area tested in square centimeters, and d is the density in gramsper cubic centimeter.

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Fig. 5. Typical coating thickness calibration standards.

It is difficult torelate the mass of thecoating to thicknesswhen the substrate isrough or the coatinguneven. Laboratoriesare best equippedto handle this timeconsumingand oftendestructive method.

WET-FILMMEASUREMENT

Wet-film thicknessgauges help determinehow muchmaterial to apply wetto achieve a specified dry-film thickness provided that the percent of solids byvolume is known. They measure all types of wet organic coatings, such as paint,varnish, and lacquer, on flat or curved smooth surfaces.Measuring wet-film thickness during application identifies the need forimmediate correction and adjustment by the applicator. Correction of the filmafter it has dried or chemically cured requires costly extra labor time, may leadto contamination of the film, and may introduce problems of adhesion andintegrity of the coating system.The equations for determining the correct wet-film thickness (WFT), bothwith and without thinner, are as follows.Without thinner:WFT = desired dry film thickness % solids volumeWith thinner:WFT = desired dry-film thickness of solids by volume/[100%]+ % of thinner added]There are four types of gauges: notch, lens, eccentric roller, and needlemicrometer. Each has its own unique operating procedure. Notch gauges, alsocalled step or comb gauges, are most common. They are inexpensive field gauges,which can either be thoroughly cleaned after each reading and reused or disposedof. The notch-type, wet-film thickness gauge is a flat aluminum, plastic, orstainless steel plate with calibrated notches on the edge of each face. The gaugeis placed squarely and firmly onto the surface to be measured immediately afterthe coating application and then removed. The wet-film thickness lies betweenthe highest coated notch and the next uncoated notch. Notched gauge measurementsare neither accurate nor sensitive, but they are useful in determiningapproximate wet-film thickness of coatings on articles where size and shapeprohibit the use of more precise methods, such as the lens and eccentric rollergauges (refer to ASTM D 1212).The gauge should be used on smooth surfaces, free from irregularities, andshould be used along the length, not the width, of curved surfaces. Using awet-film gauge on quick-drying coatings (inorganic zinc, vinyls, etc.) will yieldin accurate measurements. ASTM D 4414 outlines a standard method for measurementof wet-film thickness by notch gauges.

THICKNESS STANDARDS

Coating thickness gauges are calibrated to known thickness standards. There aremany sources of thickness standards but care must be taken to ensure they aretraceable to a national measurement institute such as NIST. It is also importantto verify that the standards are at least four times as accurate as the gauge theywill be used to calibrate. A regular check against these standards verifies thegauge is operating properly. When readings do not meet the accuracy specificationof the gauge, the gauge must be adjusted or repaired and then calibratedagain.